1. Field of the Invention
The invention relates to multilayer sheets and films including a layer of a fluoropolymer and a layer of a thermoplastic polymer. More particularly, the invention pertains to multilayered structures having at least one layer of a fluoropolymer and having improved interlayer adhesion between a fluoropolymer layer and adjacent layers. The invention also relates to adhesives useful to adhere layers of dissimilar polymeric materials that are otherwise incompatible, and particularly to an adhesive that achieves a significantly improved interlayer bond strength between a fluoropolymer and a thermoplastic polymer layer.
2. Description of the Related Art
A wide variety of thermoplastic polymers are known, as are films formed from such thermoplastic polymers. Important physical characteristics of such films include their barrier properties, including barriers to gas, aroma, and/or vapor such as water vapor, as well as its physical characteristics, such as toughness, wear and weathering resistances, and light-transmittance. These properties are especially important in film applications such as, for example, in the use of films as a packaging material for food or medical products.
It is well known in the art to produce multilayer fluoropolymer films. See, for example, U.S. Pat. Nos. 4,146,521; 4,659,625; 4,677,017; 5,139,878; 5,855,977; 6,096,428; 6,138,830; and 6,197,393. Many fluoropolymer materials are commonly known for their excellent moisture and vapor barrier properties, and therefore are desirable components of packaging films, particularly lidding films and blister packages. In addition, fluoropolymers exhibit high thermal stability and excellent toughness. However, such use of fluoropolymers is restricted to specialty packaging applications due to their relatively high cost. A suitable means of reducing the cost of a packaging material fabricated from a costly polymer is to form multilayer structures in which the polymer film is laminated with other, less costly polymer films. This approach is particularly desirable for the fluoropolymer packaging applications since a thin layer of the fluoropolymer is often all that is needed to take advantage of the desirable properties of the fluoropolymer while minimizing the cost. However, fluoropolymers do not adhere strongly to most other polymers. In fact, most fluoropolymers are known for their non-stick characteristics. This is very disadvantageous, because poor bond strength between layers can result in the delamination of multilayer films.
To improve the bond strength between a layer of a fluoropolymer and a layer of a thermoplastic polymer (e.g. a non-fluoropolymer layer), an adhesive tie layer may be used between adjacent layers. For example, U.S. Pat. No. 4,677,017 discloses coextruded multilayer films which include at least one fluoropolymer film and at least one thermoplastic film which are joined by the use of an adhesive polymer, particularly ethylene/vinyl acetate polymers, as an adhesive tie layer. U.S. Pat. No. 4,659,625 discloses a fluoropolymer multilayer film structure which utilizes a vinyl acetate polymer adhesive tie layer. U.S. Pat. No. 5,139,878, discloses a fluoropolymer film structure using an adhesive tie layer of modified polyolefins. U.S. Pat. No. 6,451,925 teaches a laminate of a fluoropolymer layer and a non-fluoropolymer layer using an adhesive tie layer which is a blend of an aliphatic polyamide and a fluorine-containing graft polymer. Additionally, U.S. Pat. No. 5,855,977 teaches applying an aliphatic di- or polyamine to one or more surfaces of a fluoropolymer or non-fluoropolymer material layer.
As an alternative to an adhesive tie layer, a surface treatment of one or both of the layers has been used to increase the adhesive bond strength between the two dissimilar layers. For example, U.S. Pat. No. 6,197,393 describes treating a non-fluoropolymer layer by providing a bonding composition which comprises a primary or secondary di- or polyamine and a non-fluorinated base polymer, reacting these components to form an amine-functionalized base polymer, which base polymer materials may include polyamides, polyamide imides, polyether imides, polyimides, polyureas, polyurethanes, polyesters, polycarbonates, functionalized polyolefins and polyketones. This is then compounded with a second, different, non-fluorinated polymer to form a blend layer. The blend layer may then be processed with a fluoropolymer layer to form multilayered articles or structures. Additionally, U.S. Pat. No. 6,096,428 teaches the step of blending a carboxyl, carboxylate, anhydride, amide, imide, hydroxyl, or oxycarbonyl functional polyolefin with an organic or inorganic base and an organo-onium compound, forming a non-fluorinated polymeric material. This non-fluorinated material is then capable of being laminated to a fluoropolymer layer under heat and pressure, and formed into articles or structures. U.S. Pat. No. 5,855,977 teaches a multilayered structure having a fluoropolymer layer and a non-fluorinated polymeric layer that has an aliphatic di- or polyamine present.
There is a continuing need in the art for further improvements in fluoropolymer films and film structures, particularly those which provide a film structure featuring low water vapor and gas transmission, and good physical characteristics. More particularly, there is a need in the art for multilayer fluoropolymer films which include non-fluoropolymer films that have good properties that are acceptable for forming packaging and lidding films. The present invention satisfies this need in the art. The invention provides an adhesive composition suitable for obtaining excellent bond strength between a fluoropolymer layer and a thermoplastic polymer layer or another fluoropolymer layer.